Dragger option not honored when drawer is open

If you initialize your snapper with the `dragger` option you will indeed not be able to drag your drawer open using any other element. However, once the drawer is open you can then drag anywhere on the content element.

Example:
http://jsbin.com/tulavo/1

Code:
http://jsbin.com/tulavo/1/edit

Steps to reproduce:
1. Drag the drawer open using the navbar (e.g. reveal left drawer)
2. With drawer open drag content (not navbar) to open drawer on the other side (e.g. right drawer).

Reproduce the behavior with the linked JS Bin example and edit page, following the listed drawer and content drag steps. Trace Snap.js's dragger handling for an open drawer; done means content cannot initiate the opposite drawer when the dragger option restricts dragging to the configured element.

Written by the indexing model from the issue text.